Production of metals



Jan. 1, 1957 J. H. MOORE PRODUCTION OF METALS 2 Sheets-Sheet 1 Filed Jan. 22, 1952 FIG.

Oxygen Ni+rogen fiftogen qCarbon m+coubom 20 combo IO 20 3O 4O 5O 6O 7O 80 90 I00 Time Minui'cs 2 G Cl r U yi P U O98765432| J0.0 00 00On- 00000000 o 40 so I00 I20 I40 I60 I 200 d(C O) in Micron fi' /min/Lb of Iron IN V EN TOR. JAMES H. MOORE cmw/W ATTORNEY Filed Jan. 22, 1952 J. H. MOORE PRODUCTION OF METALS 2 Sheets-Sheet 2 .lO F '09 .07

B *Maximum Pumping Capucii'y t .06 3 D C .05 f FIG. 3 :5, .04 0 Minimum Pumping Copacii'y 3 \l .03 I s I (I! I r .02 I/ O I .Ol

\IX/XI' \I [O00 I00 IO Pressure in Microns Pumping Vacuum- Sys'rem F 28 urnace Timer 4 CircuH' J r\ Alpha+ron 'Rofafqble Scale Q6 3 Valve Closing 4 and Tlmer Sfuri' Conluc'l' INVENTOR. l I JAMES H. MOORE Arres'rinq Mechanism or Signql BY UA A w. N97;

ATTORNEY United States Pater-t io M 5 PRODUCTION 0F METALS James H. M0ore, -Swampscott, 'Mass., assign-or to NationalResearch-Corporation, Camhridge, Mass., a corporation of Massachusetts -Applic'ation January 22,1952, Serial No.- 267,670

8"Claims. -(Cl. 75--49) "This" inventionrelates to the production of metals and more particularly to the produotion' ofhighpurity iron and iron alloys.

The production ofiron and certain iron alloysof very highpurity has 'hitherto'been limited in'scale' because of the 'difiiculty of eliminating contaminationby the atmosphericgases. It'has' long been re'cognizedthat nitrogen and hydrogen "may be readily removedifrom iron by heating'the iron in a high vacuum. 'However,the problem'of removing oxygen is "considerably more involved. Theaddition of deoxidizing elements, such as aluminum and silicon, can reduce'the oxygen in solution, but such additions maybe prohibitive because of'lthe amount "of the deoxidizing element required to be'left in solution at equilibrium to assure the necessary degree of deoxidation. Thus, for example, the amountsof aluminum, carbon, or silicon in solution in irongdnequilibriuur with an oxygen content of 0.005%, are-approximately- 0.-%,-U-0.5-5%, 1.5%, respectively. ln-the case of-one of thedeoxidants (i. e., carbon) it has been previously proposed that a low oxygen content could be obtainedinironanddronsilicon alloysbytheuse of vacuum-melting techniques.

In'this proposed prior-art methodan accurate oxygen and-carbon analysis-is first 'madeof the charge and then either one'or the other ofthese elementsis-added=to stoichiometrically' balance the 7 analysis for the reaction of carbon 'andoxygen to'form carbon'monoxide. The

charge is then melted in a vacuum for-asuflicient-"time for the pressureto reach a low value after the'initial pressureincrease which is due to the carbon monoxide formation resulting from the reactionbetween oxygen and carbon inthe melt. This low valuehas-been said to be preferably l0imicrons (0.0l mm.) or less, low

pressure being obtained in-from one to twelve' hours.

This'technique is satisfactory if extremely'accuratemnalyses of the oxygenand carbon contents'of:thetiron'are possible, and also if nocontamination of the molten iron is introduced by reaction between the molten iron and the crucible. However, it is extremely difficult to obtain an accurate carbonand oxygen analysis"-for -'the charge in those cases where' it is 'difliculfl-to obtain a representative sample, such as-when the-'pieces of' iron vary in shape and degree of oxidation, for example'when electrolytic flake is'used as a source of the'ironto be purified. In large charges where several piecesof iron are being purified in a single melt, an absolutely representative sample is almost impossible to obtain.

While there are several types 'of crucible materials which do nothave "an' appreciable reaction with"molten iron under vacuum at temperatures oftheio'rder'of 1600 C. and above, these crucible'materials, such aszirconia or beryllia, are extremely expensive and cannot be employed for producing pure iron in commercial quantities. The normallyemployed commercial refrac'ztories, such as alumina, silica, zircon and magnesia, are unsatisfactory from the standpoint of contamination whenutilized'with molten iron in a vacuum furnace unless" they "are emplayed in accordancewith the presentinvention.

2,776,204 Patented Jan. 1, 1957 ice 'Of the above mentioned commercially available'refrac- 'tories, frnagnesia is probably the -'most interesting. At "s teel making temperatures, and'at atmospheric pressure, it is verystable -andgives rise to no contamination in the melting of iron. Furthermore, "its resistance to thermal shock is good. 3 However, when one considers the stability of magnesia at'low-pressures, itis 'evidentfrom' thermochemicalcalculations that,''at"-equilibrium, the degree of contamination by oxygen w'ill'be considerable by reason bfthe volatility'ofmagnesium which drives the reduction Qf MgO ,bymolten'iron. Thus, it would appear that.it is impossible to holdironinrnagnesia at'low pressures for extended-periods of'time without contamination in excess'of 0.1"% -'oxygen',because. of the unfavorable equilibrium. This theoreticaljjprediction has been verified experimenta'lly. Howevenithepresent invention permits the use of such'crucibles to obtain high purity iron.

Accordingly," a principle object of 'the present invention "is ito' provide a method for; producing high purity 'iron and iron 'alloys' which eliminates the needfor crucible material which is expensive and scarce.

.Another object ofithe. invention-is to provide aprocess of. the above ,type which employs a crucible. material having a metallic constituent whichistoo volatile to'be retained in' the iron bath.

Still another object vof Y the invention. is tov .provide such a. process which employs. a dynamic purification .to

prevent. oxygen vcontamination of..the .molten iron-despite an unfavorable equilibrium between themolteniron and -L the material of :the. crucible.

Still. another. object ofothe .inventionds to provide such a-process whichiobviatesthe necessity for accuratecarbon L and oxygen. analyses .ofthechargepriorto melting thereof.

-taileddisclosure, :and the scope'of'theapplicationof which will berindicated in'theclaims.

1 For: a fuller understanding of rthe nature and objects of the rinventiomzlreferenceshould be had'to the'following detailed "description" taken a inconnection with the ac 'companying drawings wherein:

i Fig. l:is agraphEshowing oxygencarbon, nitrogen and hydrogen :Tcontent as a"function of time during vacuum refining -of iron;

FigL Z is a 'graph showing-oxygen content as a function of carbon:monoxide evolution rate;

Fig. 3 is a--graphshowing oxygen "content as a function of pressure-foravacuum system whose time of pumpdown is varied; and

Fig. 4 is aSchematic, diagrammatic illustration of one preferred" measuring a'nd control system-embodying the present invention.

In the present inventionhignpurity iron having less thanabout' .01'5%"oxygen is obtained'by a process which involves the'use-of relativelyinexpensivecrucibles which predominate in magnesia. In practicing the present .inventiorr a rough calculation of*thecontained oxygen in the iron charge is' first-"obtained andthen an excess of carbon' ('over'that stoiohiometrica'lly necessary for re action withnhe" oxygen) is'added "to the charge. The

furnace is then pumped down to a low total pressure on the order of 0.001 mm. and the charge is heated up to a temperature on the order of about 1650 C. As the charge reaches this high temperature the reaction of carbon and oxygen to form carbon monoxide will proceed at a relatively high rate, causing the pressure in the vacuum system to rise correspondingly, depending upon the pumping capacity of the system. As the rate of carbon monoxide evolution falls, due to the deoxidation of the iron, the pressure in the system also falls. However, the pressure, in itself, is not a reliable measure of the carbon monoxide evolution rate or of the carbon and oxygen balance in solution which corresponds to a given carbon monoxide evolution rate. This is due to the fact that the pressure is affected by the design and condition of the pumping system, by the weight of metal in the charge, by the contamination of the system, and by numerous other variables which cannot be adequately compensated for in commercial practice. It is, therefore, not significant to state that the attainment of a pressure of ten microns, for example, is indicative of a certain carbon and oxygen balance in the melt from one day to the next, or even from one melt to the next. Furthermore, while the carbon and oxygen contents are still relatively high, the oxygen content serves as a limitation on both the equilibrium and kinetics of the reaction of iron with magnesium, and the carbon reacts rapidly with any added oxygen from the crucible reaction. However, when the carbon and oxygen in solution get quite low, there comes a point beyond which the carbon has reduced the oxygen in solution to so low a value that diffusion in the boundary layer next to the crucible, and subsequent distribution of oxygen by molten currents, becomes more dominant than the deoxidation effect of the carbon. The oxygen content of the melt then begins to rise in a relatively rapid approach toward the equilibrium which exists between the molten iron and the crucible material.

This general behavior can be well illustrated in Fig. l in which carbon, oxygen, nitrogen, and hydrogen contents are plotted as a function of time during which a charge of iron was held at a relatively constant temperature of about 1650 C. From this Fig. 1 it may be seen that while the carbon content decreases rapidly, the oxygen content also decreases rapidly. However, at a certain time a minimum in oxygen content is obtained. Beyond this time the oxygen content begins to increase quite rapidly. Meanwhile, the nitrogen and hydrogen contents have fallen to very low values.

It has been found that the approach to this minimum oxygen content, illustrated in Fig. 1, is marked by a rapid decrease in the rate of evolution of carbon monoxide. Furthermore, when the melting practice is standardized with respect to the amount of carbon added to insure an excess over the oxygen content, and the temperature is controlled closely, the rate of carbon monoxide evolution, at which the minimum oxygen content is reached, is relatively constant. If the evolution rate is measured at short time intervals, as more fully described hereinafter, this critical rate may be readily detected and the reaction between the crucible and the molten charge may be arrested. This arresting of the reaction may be achieved in several ways. In one way the charge may be poured from the crucible into a suitable mold. In another, the vacuum furnace may be flooded with a relatively high pressure (such as an atmospheric pressure) of an inert gas such as argon, helium, or the like. This atmospheric pressure of the argon will suppress the iron-magnesia reaction.

Referring now to Fig. 2 there is shown a curve wherein the weight percentage of oxygen in the iron is plotted against the carbon monoxide evolution rate in micron cubic feet per minute per pound of iron. When the carbon monoxide evolution rate is above 40 micron cubic feet per minute per pound of iron, the weight of contained oxygen can be very substantial. However, as the carbon monoxide evolution rate falls to about 20, the oxygen content in the iron has fallen drastically to below .02. From this point the carbon monoxide evolution rate continues to fall, as does the oxygen content of the iron, until a carbon monoxide evolution rate of about 4 micron cubic feet per minute per pound of iron is reached. At this point the carbon has been almost entirely used up and the equilibrium reaction between the iron and the crucible begins to be the controlling factor in the oxygen content, and as the carbon monoxide evolution rate decreases further the oxygen content drastically increases. As a consequence, in the practice of the present inven tion the iron is poured, or the crucible reaction is otherwise stopped (such as by flooding with argon) after the time when the carbon monoxide evolution rate has dropped to below about 20 micron cubic feet per minute per pound and before the time when the carbon monoxide evolution rate has fallen to about 4 micron cubic feet per minute per pound.

In a preferred embodiment of the invention this criti cal evolution rate is set between about 18 and 8 micron cubic feet per minute per pound of iron. This is due to the fact that it is desired to assure oxygen contents of less than about .015 as a higher limit. The lower limit is set by the fact that there is an appreciable time lag during the determination of the carbon monoxide evolution rate. If it is attempted to reach the absolute minimum carbon monoxide evolution rate, it is quite possible that the crucible reaction may be the controlling factor because of the time that the carbon monoxide rate determination requires.

In one specific embodiment of the invention the following procedure is followed.

Example I A charge of iron, in the form of 30 lbs. of carbonyl L iron stock, is added to a magnesia crucible. To this iron charge there is added approximately 12 grams of carbon in either massive or powder form, but preferably the former. The furnace containing the crucible is then evacuated to a low free air pressure on the order of .00l.0l0 mm. Hg abs. When this low pressure is obtained the iron charge in the crucible is heated as rapidly as possible, such as by the use of an induction coil, to a preferred refining temperature on the order of about 1650 C. As a result of this heating a pressure rise is caused by the very rapid evolution of carbon monoxide. When the temperature has leveled off at about 1650 C., carbon monoxide evolution rates are determined about every five minutes. This reading may be readily obtained by valving off the pumping system and measuring the pressure rise. This pressure rise is directly related to the carbon monoxide evolution rate (E) and can be calculated from the following formula:

where n is the pressure rise in microns Hg abs. per minute, V is the effective volume of the vacuum system in cubic feet, and W is the weight of the iron charge in pounds.

When the evolution rate (E) has fallen to less than about 20 micron cubic feet per minute per pound of iron, preferably less than about 18 micron cubic feet per minute per pound of iron, the charge may be poured into a suitable mold therefor if pure iron is to be made. If an alloy is to be made the vacuum furnace may be flooded with an atmosphere of inert gas when the critical carbon monoxide evolution rate has been reached.

Example II The operating procedures are identical to those described in Example I except that, after initially pumping down the system to a low free air pressure on the order of .O0.0l0 mm. Hg abs., an inert gas pressure on the order of mm. Hg abs. is introduced into the system The -same' techniques -.are' employed ayin Example I. However, when the critical low carbon monoxide evolultion rate has been reached an inertgas-pressure of-.;between- 50 mm. and 7 60 mm. Hg abs-n ofargon is introduced into the vacuum furnace. There 'are' 'then added to the molteniron the add'itions necessary to give a -final steel-.-compos'ition within the r oll'owing composition range based'on' the weight'o'f the iron-charge:

0.95 1110 wt; percent carbon "0.250.45 wt. percent manganese 0.005 "wt. p'ercent'maxim'umphosphorus or sulfur 0.20 0.3S'wt':percent silicon 1'13 -11'60wttpercentchromium Balance iron i This chargeiis-then stirredpsuchas by lu'senofvinduced currents, until the :addition'elementstare.=thoroughly.zmixed rin-totthe melt. -This stirring=may=be continuedtor about -5-.-1.0minutes. This proceduretwill; give asteel which corresponds closely with S.-AnEa-521001Steel. Byypracticing the-present-inven-tion 'suchtia-steelwhasabeen found to have oxygenand 7 nitrogen t contents -.on the order not 0;005%i and 0.0005%,-re spectively. Suchzatsteeluhaving .this 1ow,-gas content, either-.dissolvedor in.-the.fiorm-.of inclusions, has been .found --tohavel --superiorfatigue .strength characteristics tor .applic'ation int-bearings tend 'the like. :Bytheuseaof this inventionathex-quantitieswof the .other alloying elements, such as:carbon,.-can be. main- .tained within. much: narrower ranges. than thosemormally specified for such alloys .soasato provide moreiuniform response to heat treatment and atsmaller preadsin physii'c'al'properties.

While the principles of the invention havebeen-described .specifically above inconnection with-..-the preferred embodiments thereof, it wouldrseemtadvisable to point out, in more detail, the critical dynamic-relationship of the invention as distinguished from .the static purification. systems of. the. prior .art discussed earlier in the specification. For this purpose. reference. shouldbe had'to Fig. 3 wherein four curvesf'oftot'al oxygencontent in molten iron arefplottedagainst.total pressurein microns. Of these curves,'curve A "showsasystem where there is a maximum pumping capacity (i. e.,there.is.-a large, high-vacuum pump which is voperating. attfull..ca *pa'city with a clean vacuum-systemtwhich'is=;not.outgass ing). As can be seen fromcurve A, the minimum oxygencontent occurs at a low total pressure-fof lessthan about 1.5 microns; Curve Billustrate's the situation when the pumping capacity is' somewhat decreased; for .ex-

ample due to deteriorated pump voil. "In curve "B \the minimum oxygen content correspondsto a, total pressure ofapproximately 12. microns. With'curvelCQWhich represents an evenlower pumping capacity, the minimum oxygen content occurs at approximately 20 microns, while "theoxygen content at a pressure of. 1%. microns is on the "order 'ofabo'ut 1%, or the equilibrium between the iron and the magnesia crucible. The same situation. is true with respect to curvefD which represents a minimum pumping capacity.

Fromlan examination of the curvesin. Big. 3 it should be readily apparent that staticdeterminationtof .the.oxygen content as a function of total pressure. inv theivacuum furnace isicompletely useless unless it'.is.. known.exactly which curve isactually being utilized. duringLthe pump- .down. As explainedpreviously, variations irr2the.sizeof the melt in addition. to'the humidity "of: thelatmosphere, cleanliness of the interior of the vacuum. furnace,.c1e anlimess .ofthe, pumpoil,.andinumerous other variables make it enormously .di'fiicult -topredictltl1e' pumping capacity of'Lthe systematany given time. However, it isquite simple to measurethe carbon monoxide evolution rate andtoutilizethis'dynamic standard as the control'for determining oxygen content.

.Referring now to'Fig;-4 there is shown a diagrammatic illustration of a vacuum furnace pumping system, and related co'ntrol eleme'nts, arranged for practicing the present invention. f In this Fig." 4 the vacuum furnace is indicated at 10' and has associated therewith a suitable pumping system' 12. "This pumping system may be of the'type normal1y"utilized in .vacuum. metallurgy, and "may include. afdiffusion pump "in combination with a mechanic'alvacuum pump. Between. the furnace 10 and the pumping system 12 there'is situated a valve'14 which 'can 'be' operated"to-isolateithe vacuum furnace from the :pumping'system. "Thepressure in the vacuum furnace is arrangedto be read by 'an' Alphatron 16 of the type ".disclosed' in U S- Patent No; 2,497,213. Other 'suitable pressuregaugesmay equally be employed.

Valve-His 'arranged'to' be closed by a time-roirc'uit '18,this timer circuifbeingiclosed' by a valve-closing and timer starting "circuit 20 which can be automatically or "mannally'operated'=at'predeterminedperiods of time to 'isolatethe'vacuum furnace from the pumping system and 'tomeasurethe'time duringwhichthis isolation is accomplished. The output signal from the Alphatron PFCS'SUTBT'gIUgfi 16is fed' toanindicating nee'dle'22, the positiom'of'v/hich;withrespect to a rotatable scale'23, is controlled bythepressure in'the vacuum furnace 1t) "as-retithby the Alphatronl6. Carried by the rotatable 'scal'e =23 is' as'egmental elemenfzt which, when contacted by the needle22; is arranged toclose a'co'ntact circuit 30. The needle 22"is"-arranged so'that it will normally pass over thesegme'ntal contact =24 without engaging this con- "taof'24. However, "when solenoid 26' isoperatedlby the timer circuit- 18," theneedle is pressed downwardly against the dial 23. If the needle 22 should be over the contact elemenfl-M, the contacccircuit 30 will be closed. The 'rot atable scale 23 isj'preferably 'alsosuitably graduated so that it will 'read directly the carbon monoxide evolution "rate inmi'croncubic feet per minute per pound of iron "being melted in the'vacuum furnacelt).

""The contact-circuit 30 is preferably arranged so as to actuatean arresting'mechanism 32 associated withthe vacuum furnace or a signalwhich will indicateto the furnace operator that a predetermined carbon monoxide 'evolution'rate has-been obtained. The control circuit also ineludes}in a preferred embodiment, an automatic -'zero-set mechanism 34 which will align the 0 on the scale' 28wi-th the needle '22 at the beginning of the timing circuit. This alignment can, of.course, be made manually at the time the timing cycle is'commenced.

Referring now to the operation 'of'Fig. 4, when it is desired to measure the carbonmonoxide evolution rate in "the furnace? the operator actuates the valve closing and timingstartercircuit 20. This immediately a'ctuates'the -'zero set mechanism 24 so that,*"a-t zero. time, the 0 indi- "catiomon the"scale*28 is positioned under the needle 22. Thetimer circuit-18 also is started, this timer circuit 'cIosing the' valve"-14. The vacuum=iurnace 10 is now 'isolatecl'from the pumping system, and the pressure therein begins to rise "due to carbon monoxide evolution. This pressure rise is're'ad by the Alphatron 16,'thus causing 'the-needle 22 to move to' the right across the face of the'scale23. Ifthe carbon monoxide evolution rate is high -the"ne'edle22 will move rapidly. When the solenoid 26 isac'tuated'atthe end of a. predetermined time which, for convenienceg'may' be one minute, it will press the needle22. against'the rotatable scale 23. Since the needle has been ,moving rapidly, it will have passed the contact elementl '24..and' the contact circuit 30 will not be energi-zed. Valve 14 is-then :opened, either "automatically or :manually a-nd the -vaouu-m. pumping system ,isused to again reduce the pressure in the furnace. Another reading is taken at an interval of about five minutes, either automatically or manually, and the measuring cycle is repeated. The operation is repeated until such time as the carbon monoxide evolution rate has dropped below about 20 micron cubic feet per minute per pound of iron. When this low carbon monoxide evolution rate has been achieved, the movement of the needle 22, with respect to the rotatable scale 23, will be relatively slow. Thus, when the solenoid 26 is energized to indicate the end of the measuring cycle, the needle 22 will be pressed against the contact element 24 and the contact circuit will be closed. This will energize either the signal system or the arresting mechanism depending upon whether or not the system is completely automatic. When the signal circuit has been energized, the operator may decide to continue to pump down the system if the needle indicates that the carbon monoxide evolution rate is still around 18 micron cubic feet per minute, and may take another reading a few minutes later so as to strike the lowest possible point on the oxygen curve. When the desired carbon monoxide evolution rate has been obtained, the operator may flood the tank with argon, if desired, or may pour the charge from the crucible into a suitable mold therefor. Alternatively, he may adjust the contact element 24 to energize the arresting mechanism at a lower evolution rate than 20 micron cubic feet per minute per pound. The arresting mechanism may be either a gas flooding system or a pouring mechanism, the former being preferred when alloys are to be made.

Numerous other control circuits may be employed in lieu of the one specifically illustrated, and numerous other pressure measuring techniques may be utilized. While direct indicating and direct reading instruments are naturally preferred, other techniques for determining the carbon monoxide evolution rate in any instant of time can be substituted without departing from the spirit of the invention.

When additions are to be made to the molten iron charge, it is preferred that a furnace of the type shown in my copending application Serial No. 187,016, filed September 27, 1950, now U. S. Patent No. 2,625,719, be employed. This furnace has the additional advantage that the next charge of iron may be added to the furnace without opening the furnace interior to the atmosphere and thus any residual iron in the crucible is kept free of air oxidation.

Since certain changes may be made in the above product, process, and apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawing, shall be interpreted as i1- lustrative and not in a limiting sense.

What is claimed is:

l. The process of manufacturing high purity iron and alloys of iron having an oxygen content by weight of less than about 0.015% and a carbon content by weight of less than about 0.005%, said process comprising the steps of confining said iron in a vacuum furnace in a crucible predominating in an oxide of a metal having a high vapor pressure at the temperature of molten iron, adding to said iron, at some stage in said process, an amount of carbon in excess of the amount needed for stoichiometrically reacting with the oxygen in said iron, evacuating said furnace, heating said iron and carbon to a temperature on the order of about 1650 C., maintaining said temperature substantially constant, measuring the rate of evolution of carbon monoxide from said molten iron resulting from reduction of iron oxide by said added carbon, and pouring said iron into a mold between the time when the carbon monoxide evolution rate falls to about 20 micron cubic feet per minute per pound of iron and before the carbon monoxide evolution rate falls to about 4 micron cubic feet per minute per pound of iron.

2. The process of manufacturing high purity iron and alloys of iron having an oxygen content by weight of less than about 0.015%, said process comprising the steps of confining said iron in a vacuum furnace in a crucible predominating in an oxide of a metal having a high vapor pressure at the temperature of molten iron, adding to said iron, at some stage in said process, an amount of carbon in excess of the amount needed for stoichiometrically reacting with the oxygen in said iron, evacuating said furnace, heating said iron and carbon to a temperature on the order of about 1650 C., maintaining said temperature substantially constant, measuring the rate of evolution of carbon monoxide from said molten iron resulting from reduction of iron oxide by said added carbon, and arresting the reaction between the molten iron and the crucible at some point between the time when the carbon monoxide evolution rate falls to about 20 micron cubic feet per minute per pound of iron and before the carbon monoxide evolution rate falls to about 4 micron cubic feet per minute per pound of iron.

3. The process of manufacturing a high purity iron alloy having an oxygen content by weight of less than about 0.015%, said process comprising the steps of confining said iron in a vacuum furnace in a crucible predominating in an oxide of a metal having a high vapor pressure at the temperature of molten iron, adding to said iron, at some stage in said process, an amount of carbon in excess of the amount needed for stoichiometrically reacting with the oxygen in said iron, evacuating said furnace, heating said iron and carbon to a temperature on the order of about 1650" C., maintaining said temperature substantially constant, measuring the rate of evolution of carbon monoxide from said molten iron resulting from reduction of iron oxide by said added carbon, arresting the reaction between the molten iron and the crucible at some point between the time when the carbon monoxide evolution rate falls to about 20 micron cubic feet per minute per pound of iron and before the carbon monoxide evolution rate falls to about 4 micron cubic feet per minute per pound of iron, and adding at least one alloying element to said deoxidized iron to form a high purity iron alloy.

4. The process of purifying iron to obtain a molten iron containing, by weight, less than about .005 carbon and less than about .015% oxygen, said process comprising the steps of confining said iron in a vacuum furnace in a crucible predominating in an oxide of magnesium, adding to said iron at some stage in said process, an amount of carbon in excess of the amount needed for stoichiometrically reacting with the oxygen in said iron, evacuating said furnace, heating said iron and carbon to a temperature on the order of about 1650" C., maintaining said temperature substantially constant, measuring the rate of evolution of carbon monoxide from said molten iron resulting from reduction of iron oxide by said added carbon, and arresting the reaction between the molten iron and the crucible at some point between the time when the carbon monoxide evolution rate falls to about 20 micron cubic feet per minute per pound of iron and before the carbon monoxide evolution rate falls below about 4 micron cubic feet per minute per pound of iron.

5. The process of purifying iron to obtain a molten iron containing, by weight, less than about .005% carbon and less than about 015% oxygen, said process comprising the steps of confining said iron in a vacuum furnace in a crucible predominating in an oxide of magnesium, adding to said iron at some stage in said process, an amount of carbon between about 25% and 50% in excess of the amount needed for stoichiometrically reacting with the oxygen in said iron, evacuating said furnace, heating said iron and carbon to a temperature on the order of about 1650 C., maintaining said temperature substantially constant, measuring the rate of evolution of carbon monoxide from said molten iron resulting from reduction of iron oxide by said added carbon, and arresting the reaction between the niolte'ri iron and the crucible at some point between the time when the carbon monoxide evolution rate falls to about 18 micron cubic feet per minute per pound of iron and before the carbon monoxide evolution rate falls below about 8 micron cubic feet per minute per pound of iron.

6. The process of claim 2 wherein the reaction between the molten iron and the crucible is arrested by creating in said vacuum furnace an atmosphere of an inert gas having a total pressure in excess of 50 mm. Hg abs.

7. The process of claim 6 wherein at least one alloying constituent is added to said molten iron after said inert gas is introduced into said furnace chamber.

8. The process of claim 2 wherein the reaction between the molten iron and the crucible is arrested by pouring said molten iron out of said crucible.

References Cited in the file of this patent UNITED STATES PATENTS 1,555,314 Rohn Sept. 29, 1925 2,036,496 Randolph Apr. 7, 1936 2,054,922 Betterton et al Sept. 22, 1936 2,071,942 Rohn Feb. 23, 1937 2,076,800 Thummel Apr. 13, 1937 2,155,349 Mathesius et al. Apr. 18, 1939 2,513,935 Harris July 4, 1950 2,587,793 Waldron Mar. 4, 1952 FOREIGN PATENTS 40,649 Norway Dec. 22, 1924 OTHER REFERENCES Yensen, volume XXXII, Transactions of the American Electrochemical Soc. (1917), pages 178 and 179. 

1. THE PROCESS OF MANUFACTURING HIGH PURITY IRON AND ALLOYS OF IRON HAVING AN OXYGEN CONTENT BY WEIGHT OF LESS THAN ABOUT 0.015% AND A CARBON CONTENT BY WEIGHT OF LESS THAN ABOUT 0.005%, SAID PROCESS COMPRISING THE STEPS OF CONFINING SAID IRON IN A VACUUM FURNACE IN A CRUCIBLE PREDOMINATING IN AN OXIDE OF A METAL HAVING A HIGH VAPOR PRESSURE AT THE TEMPERATURE OF MOLTEN IRON, ADDING TO SAID IRON, AT SOME STAGE IN SAID PROCESS, AN AMOUNT OF CARBON IN EXCESS OF THE AMOUNT NEEDED FOR STOICHIOMERTRICALLY REACTING WITH THE OXYGEN IN SAID IRON , EVACUATING SAID FURNACE, HEATING SAID IRON AND CARBON TO A TEMPERATURE ON THE ORDER OF ABOUT 1650*C., MAINTAINING SAID TEMPERATURE TURE SUBSTANTIALLY CONSTANT, MEASURING THE RATE OF EVOLUFROM REDUCTION OF IRON OXIDE BY SAID ADDED CARBON, AND POURING SAID IRON INTO MOLD BETWEEN THE TIME WHEN THE CARBON MONOXIDE EVOLUTION RATE FALLS TO ABOUT 20 MICRON CUBIC FEET PER MINUTE PER POUND OF IRON AND BEFORE THE CARBON MONOXIDE EVOLUTION RATE FALLS TO ABOUT 4 MICRON CUBIC FEET PER MINUTE PER POUND OF IRON. 